1. Field of the Invention
This invention relates to a disc drive unit having a skew regulation mechanism for correcting the inclination, if any, of the optical pickup thereof relative to the optical disc and also to a disc recording and/or reproducing apparatus comprising such a disc drive unit.
2. Description of the Related Art
Known optical discs include CDs (compact discs) and DVDs (digital versatile discs) as well as MOs (magneto-optical discs) and MDs (mini discs). A variety of disc recording and/or reproducing apparatus have been marketed for such discs and disc cartridges containing discs of the sort under consideration.
FIG. 1 of the accompanying drawings schematically illustrates a known disc drive unit 200 of a disc recording and/or reproducing apparatus adapted to record information signals onto or reproduce information signals from an optical disc. Referring to FIG. 1, the known disc drive unit 200 comprises a disc rotary drive mechanism 201 for driving an optical disc to rotate, an optical pickup 202 for carrying out an operation of writing signals onto or reading signals from an optical disc that is driven to rotate by the disc rotary drive mechanism 201 and a pickup feed mechanism 203 for driving and feeding the optical pickup 202 to move in a radial direction of the optical disc. The above listed components of the disc drive unit 200 are fitted to a base 204.
The disc rotary drive mechanism 201 includes a flat spindle motor 206 that is equipped with a turntable 205 for holding an optical disc. The spindle motor 206 drives the optical disc to rotate integrally with the turntable 205.
The optical pickup 202 writes signals onto or read signals from the optical disc by converging the beam of light (laser beam) emitted from a semiconductor laser by means of an objective lens 207, irradiating the converged beam of light onto the signal recording surface of the optical disc and detecting the returning beam of light reflected by the signal recording surface of the optical disc.
The pickup feed mechanism 203 includes a pair of guide shafts 208a, 208b that support the optical pickup 202 so as to be movable in a radial direction of the optical disc, a rack member 209 fitted to the optical pickup 202, a lead screw 210 to be engaged with the rack member 209 and a stepping motor 211 for driving the lead screw 210 to rotate. As the stepping motor 211 drives the lead screw 210 to rotate, it consequently drives the rack member 209 that is engaged with the lead screw 210 to be displaced with the optical pickup 202 in a radial direction of the optical disc.
The base 204 has a table aperture 212a for exposing the turntable 205 and a pickup aperture 212b for exposing the optical pickup 202. The two apertures 212a, 212b are continued from each other. A number of components including the spindle motor 206, the opposite ends of the pair of guide shafts 208a, 208b, the lead screw 210 and the stepping motor 211 are fitted to the main surface of the disc drive unit that is opposite to the other main surface where the turntable 205 and the optical pickup 202 are exposed to the outside by way of the apertures 212a, 212b. 
In the disc drive unit 200 of the disc recording and/or reproducing apparatus configured as described above, the disc rotary drive mechanism 201 drives the optical disc to rotate and the pickup feed mechanism 203 drives and feeds the optical pickup 202 to move in a radial direction of the optical disc so that the optical pickup 202 operates for writing signals onto or reading signals from the optical disc. As a result, information signals are recorded onto or reproduced from the target recording tracks of the optical disc.
For disc drive units of the above described type, efforts are being paid to record signals highly densely on an optical disc by reducing the wavelength of the beam of light to be irradiated from the optical pickup 202 onto the optical disc and increasing the numerical aperture of the objective lens 207. However, as the wavelength of the beam of light to be irradiated onto the optical disc is reduced and the numerical aperture of the objective lens 207 is increased, aberration occurs frequently when the optical axis of the beam of light emitted from the optical pickup 202 is inclined relative to the signal recording surface of the optical disc (a phenomenon referred to as skew hereinafter) to consequently and remarkably degrade the signal recording/reproducing performance of the disc drive unit. In other words, the tolerance of the disc drive unit 200 is reduced for the skew as the recording density of the optical disc increases. Therefore, it is necessary to irradiate the beam of light converged by the objective lens of the optical pickup 202 perpendicularly relative to the signal recording surface of the optical disc that is held on the turntable 205 for the purpose of improving the signal recording/reproducing performance of the disc drive unit 200.
Known techniques for correcting the inclination of the optical pickup 202 relative to the optical disc include one that is designed to regulate the fitting angle of the spindle motor 206 relative to the base 204 (see, inter alia, Patent Document 1: Jpn. Pat. Appln. Laid-Open Publication No. 2002-171709) and one that is designed to adjust the inclination of the pair of guide shafts 208a, 208b that supports the optical pickup 202 (see, inter alia, Patent Document 2: Jpn. Pat. Appln. Laid-Open Publication No. 2001-222823).
In the above described disc drive unit 200, the pair of guide shafts 208a, 208b is fitted at the opposite ends thereof to the lower surface of the base 204 typically by way of a skew regulation mechanism 220, as shown in FIG. 2.
The skew regulation mechanism 220 is adapted to support the pair of guide shafts 208a, 208b at the opposite ends of the latter so as to make it possible to move them in a direction perpendicularly intersecting the main surfaces of the base 204 and independently regulate the supported positions of the opposite ends of the guide shafts 208a, 208b in order to regulate the inclination of the guide shafts 208a, 208b. 
More specifically, the skew regulation mechanism 220 has bearing members 221 for supporting the opposite ends of the pair of guide shafts 208a, 208b, as shown in FIGS. 2 and 3. Each of the bearing members 221 has a holding hole 223 for holding a coil spring 222 and a guide slit 224 exposed to the holding hole 223 and adapted to receive the corresponding one of the end sections 208c of the guide shaft 208a or 208b. The holding hole 223 runs through the bearing member 221 in a transversal direction of the bearing member 221 and the coil spring 222 is arranged in the holding hole 223 and compressed between the base 204 and the corresponding end section 208c of the guide shaft 208a or 208b, whichever appropriate. The guide slit 224 is formed by transversally notching the bearing member 221 so as to guide the end section 208c of the guide shaft 208a or 208b, whichever appropriate, in a direction perpendicularly intersecting the main surfaces of the base 204. On the other hand, as shown in FIG. 4, the end section 208c of the guide shaft 208a or 208b, whichever appropriate, has a reduced diameter so as to be snugly inserted into the guide slit 224 and is coaxial with the guide shaft relative to the axial center S′ of the guide shaft. Referring to FIGS. 2 and 3, there is also provided a support plate 225 for pinching the bearing member 221 between itself and the base 204. The support plate 225 is equipped with a screw hole 227 for receiving an adjuster screw 226 so as to be engaged with the latter. The screw hole 227 is exposed to the holding hole 223. The front end of the adjuster screw 226 is made to abut the end section 208c of the guide shaft 208a or 208b, whichever appropriate, from the side opposite to the coil spring 222 while it is held in the state of being engaged with the screw hole 227.
With the above-described arrangement of the skew regulation mechanism 220, it is possible to change the compressed state of the coil spring 222 that is held in the holding hole 223 by adjusting the extent by which the adjuster screw 226 is driven. As a result, it is possible to adjust the inclination of each of the guide shafts 208a, 208b in such a way that the beam of light converged by the objective lens 207 is irradiated onto the signal recording surface of the optical disc perpendicularly relative to the latter by moving the optical pickup 202 in a transversal direction relative to the main surfaces of the base 204, while adjusting the positions at which the ends 208c of the guide shafts 208a, 208b are supported.
The above described skew regulation mechanism 220 is provided with a satisfactory skew margin by limiting the total of the fitting errors of the members of the disc drive unit 200 to about ±0.4 mm and considering the fitting accuracy of the spindle shaft and the turntable 205 of the spindle motor 206 fitted to the base 204 and the assembling accuracy of the optical system including the objective lens 207 arranged in the optical pickup 202. In other words, the skew regulation mechanism 220 is provided with a margin of about ±0.4 mm, by which each of the guide shafts 208a, 208b inserted into the respective bearing members 221 is allowed to move from the designed position in a transversal direction of the base 204. Therefore, the disc drive unit 200 can correct the inclination of the optical axis of the optical beam relative to the signal recording surface of the optical disc to nil if any of the spindle shaft of the spindle motor 206, the turntable 205 and the optical system including the optical pickup 202 has a manufacturing error and/or a mounting error provided that the errors are within ±0.4 mm as viewed from the design value.
Additionally, as a result of the development of high density recording optical discs in recent years realized by using a narrow pitch of arrangement of recording tracks and reducing the pit size, there are optical pickups that are adapted to operate for skew regulation on a real time basis. Such optical pickups are adapted to be subjected to an operation of skew regulation independently relative to the optical system of the optical pickup (see, inter alia, Patent Document 3: Jpn. Pat. Appln. Laid-Open Publication No. 2004-39024).
With such a skew regulation technique, the inclination, if any, of the optical pickup 202 is corrected relative to the optical disc before it is mounted on the base 204 by carrying out an operation of skew regulation in advance relative to the optical system of the pickup 202. Therefore, if a skew margin of about ±0.4 mm is provided in total for the mounting error of the component members of the disc drive unit 200 including the optical pickup 202 in the operation of skew regulation of the disc drive unit 200 that is carried out after mounting the optical pickup 202 on the base 204, the error range that is allowed to each of the component members of the disc drive unit 200 except the optical system of the optical pickup 202 is inevitably expanded. In other words, it may not be possible to eliminate disc drive units 200 of a precision level that is out of the tolerance of the assembling precision. Particularly, if the height of the entire apparatus is reduced to about 9.5 mm that is substantially equal to the height of the hard disc drive (HDD) contained in it, there arises a risk that the optical pickup 202 scrapes the optical disc and/or the bottom of the disc recording and/or reproducing apparatus when the optical pickup 202 is driven to move in a radial direction of the optical disc along the guide shafts 208a, 208b. 